Measurement of oxides of nitrogen is important in studying air pollution and its effects on the atmosphere. Combustion processes, such as from coal-fired power plants, automobiles, and the like, may produce oxides of nitrogen as a byproduct, typically as NO. In the atmosphere, NO may react with other chemicals, notably ozone (O3) to produce NO2. Combined, NO2 and NO may be referred to as “Nitrogen Oxides” or NOx. Nitrogen Oxides may combine with other chemicals in the atmosphere to produce other nitrogen compounds, which may be referred to as reactive nitrogen or NOy. Studying the effect of NOy on the atmosphere as well as its concentration levels, as well as ozone (O3) levels requires an instrument or instruments to measure the level of these chemicals in the atmosphere.
Reactive nitrogen compounds play a central role in atmospheric chemistry. Nitrogen oxides (NOx≡NO+NO2) strongly affect the oxidative capacity of the atmosphere through the catalytic cycle that produces ozone (O3) in the lower atmosphere. Total reactive nitrogen (NOy) includes NOx and all its reservoirs:NOy≡NO+NO2+NO3+2N2O5+HNO3+HONO+HO2NO2+PAN (peroxy acetyl nitrates)+aerosol nitrates+organic nitrates+ . . .
Reactive nitrogen compounds (NOy) have been identified as precursors for both ozone and fine particulate matter (PM2.5). The EPA's National Ambient Air Monitoring Strategy (NAAMS) calls for deployment of NOy monitors at approximately 75 locations. The EPA Office of Air Quality Planning and Standards (OAQPS) began an effort to gain knowledge and experience with NOy monitoring in order to resolve a number of technical issues that have been identified with NOy monitoring, as well as to be prepared to provide support to State and Local monitoring agencies as they begin to purchase, install, and operate NOy monitors. (See, e.g., McClenny, W. A., et al., Preparing to measure the effects of the NOx SIP Call—Methods for ambient air monitoring of NO, NO2, NOy, and individual NOz species, Journal of the Air & Waste Management Association 2002, 52, 542-562, incorporated herein by reference).
NOy consists of all oxides of nitrogen in which the oxidation state of the N atom is +2 or greater, i.e., the sum of all reactive nitrogen oxides including NOx (NO+NO2) and other nitrogen oxides referred to as NOz. The major components of NOz include nitrous acids [nitric acid (HNO3), and nitrous acid (HONO)], peroxy nitrates [peroxyl acetyl nitrate (PAN), methyl peroxyl acetyl nitrate (MPAN), and peroxyl propionyl nitrate, (PPN)], organic nitrates [alkyl nitrates of one or more carbon atoms and multi-functional nitrate species], and particulate nitrates.NO+NO2+NO≡NOy 
One Prior Art method of measuring NOy is the use of a thermal catalytic converter to convert reactive nitrogen species to NO followed by detection of NO by its chemiluminescence reaction with an excess of O3. NO is measured by bypassing the converter. The combination of NO2 and NOz can be then determined by the difference. This procedure is similar to the Prior Art methodology used to measure NOx, however, the catalytic converter temperature is higher in order to more completely convert NOz species, and the converter has been moved to very near the sample inlet to avoid line losses of “sticky” NOy species such as HNO3.
The NOx measurement produced by this method is not considered accurate by the research community, although it is still currently used by the regulatory community. This converter operates through reduction of NOy to NO in a heated molybdenum catalyst. When the molybdenum catalyst is operated at somewhat lower temperature, the measurement is interpreted as being NOx and not NOy (i.e., only the NO2 is supposedly converted to NO at the lower temperature). The NOy measurement may be acceptable if the heated molybdenum converter is placed very close to the end of the inlet, but the NOx measurement generally includes some fraction of the NOy compounds and is therefore considered only an upper limit to the actual NOx.
Knowledge of the abundance of this chemical family, as well as NO, NO2, and the related compound O3, is a useful indicator of total nitrogen emissions, air mass age, competition between different chemical processes, and the efficiency of ozone production associated with particular emission sources. Again, standard measurements of NOy rely on catalytic decomposition of NOy to NO, followed by NO detection using chemiluminescence (See, e.g., Fahey, D. et al., Evaluation of a catalytic reduction technique for the measurement of total reactive odd-nitrogen NOy in the atmosphere, Journal of Atmospheric Chemistry 1985, 3, 435-468, incorporated herein by reference).
The most commonly used materials for conversion are gold and molybdenum. However, catalytic converters are prone to deterioration, affecting conversion efficiencies. As a result, they require frequent calibrations and need to be “reconditioned” or cleaned periodically, depending on the history of exposure (See, e.g., Crosley, D. R., NOy Blue Ribbon panel. Journal of Geophysical Research, Atmospheres 1996, 101, 2049-2052, incorporated herein by reference).
Additionally, the chemical processes involved in the catalytic conversion are not fully understood, as illustrated by Kliner, D. A. V. et al., Laboratory investigation of the catalytic reduction technique for measurement of atmospheric NOy, Journal of Geophysical Research: Atmospheres 1997, 102, 10759-10776, also incorporated herein by reference.
Inlet design can also play a major role, as some NOy species, notably HNO3, can suffer significant losses on inlet surfaces as described by Williams, E. J. et al., Intercomparison of ground-based NOy measurement techniques, Journal of Geophysical Research: Atmospheres 1998, 103, 22261-22280, and Neuman, J. A. et al., Study of Inlet Materials for Sampling Atmospheric Nitric Acid, Environmental Science & Technology 1999, 33, 1133-1136, both which are incorporated herein by reference.
Thus it remains a requirement in the art to provide a compact and efficient instrument for measuring NOy, NO2, NO, and O3 concentrations in the atmosphere.